Sure. The constraints is that you don't get to do anything global without going through Orchard. For example, routes need to be moved to an implementation of IRouteProvider, and pretty much anything you'd put in the root global.asax or web.config should
be moved to the appropriate Orchard-ism. Http modules, being global resources, also don't work well with Orchard, and need to be refactored into filters or something else that is nicer to other modules sharing the same environment. It's usually not that much
work, and about the only issues. You then need to add a manifest for the module and it should just work. That is, unless you're doing some really funky stuff.

I have a application central of support, working with asp.net mvc 4 to convert for a module.
My module can use several tables of a external database different of the used by the other modules?
Really is possible to convert a project like this into a module?